Conventionally, known projection display devices include front projection display devices in which a projection lens is arranged on the same side as an audience viewing light beams containing information reflected from a reflective display screen, and rear-projection display devices in which a transmissive display screen is arranged between an audience and a projection lens that transmits light beams containing information to the transmissive display screen.
Among these, in rear-projection display devices, such as rear-projection televisions, it is well known to arrange the transmissive display screen in the front of a cabinet that contains a light source and the projection lens facing the screen.
Recently, various projection lenses have been proposed for use in projection display devices housed in cabinets, including those disclosed in Japanese Laid-Open Patent Applications 2002-365541, 2004-177688, and 2003-57540. In such cabinets, a smaller cabinet is preferred. Therefore, there is a demand that the volume of the cabinet be decreased. For this reason, shortening the projection distance, i.e., the distance from the surface of a projection lens that is toward the projection screen to the projection screen enables making the television thinner, but this requires widening the projection field angle of the projection lens. These considerations have led to projection lenses with widened projection field angles, as set forth for example in Japanese Laid-Open Patent Applications 2002-031754, 2004-177688, and 2003-57540.
There is a demand that cabinet-type projection displays be thin and have a low profile. In order to meet such demands, it is considered necessary to miniaturize the entire projection lens system by folding the optical path of the projection lens system by installing a deflector, such as a mirror or a prism, for deflecting the optical path of the projection lens system and reducing the projection distance, that is, the distance from the surface of the projection lens to the screen, by making the projection lens be a wide-angle lens.
Also, in projection displays of the type that form color images using multiple light valves, because a synthesizing unit is required in order to synthesize color light beams from the light valves, a projection lens having a long back focal length is demanded.
Furthermore, along with increasing the resolution of the light valves, significant improvement in the resolution of projection lenses is demanded. Degradation of resolution due to chromatic aberration is of particular concern for wide-angle projection lenses.
Therefore, because simultaneously meeting the various demands is difficult, a projection lens which satisfies the various requirements to a high degree has not yet been realized.
For example, Japanese Laid-Open Patent Applications 2002-365541 and 2004-177688, mentioned above, disclose projection lenses that provide good correction of chromatic aberration and provide a long back focal length by using a lens component with three lens elements, but the disclosed projection lenses do not have enough space for a deflector, such as a mirror, for folding the optical path, which makes it difficult to miniaturize the entire projection lens system.
On the other hand, Japanese Laid-Open Patent Application 2003-57540, mentioned above, discloses a projection lens with adequate space for a deflector. However, because the chromatic aberration correction when using a high-resolution light valve is insufficient, and because the lens element on the magnification side of the projection lens is large, it is difficult to miniaturize the projection lens system.